1. Field of the Invention
The present invention relates to a retrofocus wide-angle lens system and an optical instrument provided with this retrofocus wide-angle lens system.
2. Description of Related Art
Retrofocus wide-angle lens systems have been proposed (Japanese Unexamined Patent Publication Nos. 2007-225804, 2006-300972 and 2006-201526), as wide-angle lens systems for use in an SLR (single-lens reflex) camera, which are configured of a negative front lens group, and a positive rear lens group that includes a diaphragm, in that order from the object side, and which covers an angle-of-view of 90 through 100 degrees.
In a retrofocus wide-angle lens system, the lens diameter of the front lens group is large due to the wide angle-of-view. Hence, it is difficult to provide a normal-sized optical filter on the object side of the lens system (front lens group). Therefore it is not practical, as a product or in regard to cost, to increase the size of the optical filter in order to correspond to the diameter of the front lens group and the wide angle-of-view.
Although a method is utilized in which an optical filter is disposed on the image side of the lens system (rear lens group), a polarizing filter which is externally rotated by hand cannot in reality be used. Furthermore, in order to provide an optical filter on the image side, it is necessary to attain a long backfocus, causing the lens system to increase in size (length) and weight. Furthermore, due to the optical filter being disposed closer to the image side than the diaphragm, negative distortion that occurs in the lens elements (front lens group) that are provided on the object side of the diaphragm increase.
On the other hand, a method has been proposed (in Japanese Unexamined Patent Publication No. 2004-226740) in which an optical filter is inserted into the lens system. However, in that proposed insertion method, the incident angle of the oblique light rays with respect to the optical filter is reduced (so as to come closer to an afocal light bundle with respect to the optical filter) in order to merely avoid the influence of the angular characteristics of the optical filter.
Accordingly, it is unknown for a filter-insertion retrofocus wide-angle lens system of the related art to use an insertion filter or a built-in filter for correcting aberrations, especially distortion.